Optical metrology tools such as ellipsometers are used to characterize targets, such as semiconductor wafers, by reflecting an optical beam off of the target. Performing accurate characterization using such tools, however, presents significant challenges. For example, characterization results are affected by multiple parameters of the target. Some target parameters may be correlated such that variation of different parameters has the same effect. Correlations between target parameters make it difficult to know which parameter is responsible for a particular result. To break these correlations and accurately characterize the target based on the optical-metrology results, it is desirable to vary the manner in which the optical beam reflects off of the target.
FIG. 1 is a schematic illustration of a prior-art ellipsometer 100 for characterizing a target 120 (e.g., a semiconductor wafer). In the ellipsometer 100, an illumination system 102 provides an optical beam from a laser-driven light source (LDLS) 106 to the target 120, which reflects the optical beam. A collection system 104 provides the reflected optical beam to a spectrometer 132. The illumination system 102 includes, in sequence, a plurality of mirrors 108-1 through 108-4, a Rochon polarizer 110, an aperture 112, an apodizer 114, a convex mirror 116, and a concave mirror 118. The collection system 104 includes, in sequence, a collection mask 122 on a concave mirror 124, a convex mirror 126, a retarder 128, and an analyzer 130 (e.g., a polarizer).
The collection mask 122 includes three shuttered apertures at fixed angles of incidence with respect to the target 120 (as measured from an axis perpendicular to the surface of the target 120): 59°, 65°, and 71°. By opening one of the shutters, the collection mask 122 allows a portion of the optical beam at a specified angle of incidence to be reflected by the concave mirror 124 to the convex mirror 126, and thus to proceed toward the spectrometer 132. In this manner, the collection mask 122 masks the optical beam reflected from the target 120. The ellipsometer 100 thus allows measurements to be taken at three different angles of incidence, which is useful for breaking correlation to achieve accurate characterization of the target 120.
The collection mask is very sensitive to misalignment, however: it is situated at a position where the optical beam diverges by over 10°. And because the three apertures on the collection mask 122 are fixed, the user cannot vary any of the apertures. For example, the user cannot select other angles of incidence, cannot vary the range of angles of incidence for a given aperture (e.g., to increase beam brightness), and cannot rotate the apertures to accommodate variation in the angle of azimuth of the target 120 with respect to the optical beam.